Add some comments for VNDK / vendor snapshots

Also some files are refactored:

- snapshot_prebuilt.go is separated from vendor_snapshot.go. Now
vendor_snapshot.go contains snapshot generation codes, while
snapshot_prebuilt.go contains module definition codes.

- Some helper functions are moved from snapshot_utils.go to util.go.

- Some ambiguous names of types and functions are renamed.

We still can add more detailed comments about the snapshots. They are to
be uploaded in follow-up changes, to avoid making this change too big.

Bug: 173474311
Test: generate vndk and vendor snapshot
Change-Id: I18fa837ccdf44a042b7a78e5c3df25fd2de96d95
diff --git a/cc/Android.bp b/cc/Android.bp
index 88104e2..33f3db2 100644
--- a/cc/Android.bp
+++ b/cc/Android.bp
@@ -31,6 +31,7 @@
         "sanitize.go",
         "sabi.go",
         "sdk.go",
+        "snapshot_prebuilt.go",
         "snapshot_utils.go",
         "stl.go",
         "strip.go",